High torque inhibitor

ABSTRACT

Apparatus for inhibiting high torque created by the application of reverse voltage to a conventional motor with a field coil and armature operating in the opposite direction comprising a pair of drive resistors in series with the motor, a corresponding normally open contact connected in parallel to each respective drive resistor, an energizing coil corresponding to each contact, a timing circuit and an inhibitor circuit. During high speed forward operation, both coils are energized, closing both contacts, shorting out both drive resistors and maximizing motor voltage. When reverse operation is desired, a reverse voltage is applied across the armature, resetting the timing circuit and activating the inhibitor circuit. The inhibitor circuit will deenergize the coils, opening the contacts and remove the short across the drive resistors and thus reduce the reverse voltage being applied and permit gradual deacceleration. When armature speed nears zero, the inhibitor circuit deactivates and the energizing coils reenergize, increasing reverse motor voltage upon the expiration of the time delay of the timing circuit.

BACKGROUND OF THE PRESENT INVENTION

I. Field of the Present Invention

This invention relates generally to an apparatus for controlling thespeed of a motor and more specifically, this invention relates to anapparatus which inhibits the application of high torque when a motoroperating at high speed in a first direction is switched to run inreverse direction.

II. Description of the Prior Art

In certain lift trucks, particularly in those lift trucks known as"walking trucks", the speed is controlled by varying the resistance inthe DC motor circuit. Such lift trucks have three discrete speeds:first, second and third, depending on the series resistance to the DCmotor.

When switching from forward to reverse direction is desired, theoperator will often attempt to accomplish such a result by "plugging"the motor. "Plugging" commonly refers to reversing the direction ofcurrent in the field of a motor while it is still operating in theopposite direction. When the operator of such walking trucks shifts fromthe highest forward speed to reverse, a very high torque is imposed uponthe unit. Such high torques have been known to cause loads carried bythe trucks to spill and have caused the operators to fall from the truckitself thus causing injuries.

In order to avoid the high torque imposed upon a series resistorcontrolled DC motor type truck upon shifting from high speed forwarddirectly to reverse, some technique must be employed to controldeceleration of the DC motor armature from its velocity in the forwarddirection to zero velocity and thereafter to control acceleration of theDC motor in the opposite, or reverse direction. In a first attempt tosolve this problem, the present inventors employed a circuit whichsensed a change in direction of armature voltage, thus indicating thatthe operator intended to change direction of the truck. Upon sensing achange in direction in the armature voltage, previous control apparatusincluded a timing circuit which precluded reverse operation of the truckuntil a predetermined time had elapsed. This solution proved to be lessthan satisfactory because once the time delay had expired, the motorarmature would be immediately subjected to maximum decelerating torque,which would result in an abrupt change similar to the undesired hightorque problem. Another drawback to this solution was that when thetruck was shifted from reverse to forward, the timing circuit preventedsuitable acceleration of the truck.

SUMMARY OF THE INVENTION

Apparatus for inhibiting the high torque during the plugging of aseries-resistor controlled motor comprises a conventional truck motorwith a field coil and armature, a pair of drive resistors in series withthe motor, corresponding normally open drive resistor contacts inparallel to their respective drive resistor, an energizing coilcorresponding to each drive resistor contact, a timing circuit and aninhibitor circuit. When high forward speed operation is desired, bothcoils are energized, which closes both drive resistor contacts, shortingout the drive resistors and maximizing motor voltage. When the motor isswitched to operate in the reverse direction, the current in the fieldcoil is reversed, causing a reversal of the counter EMF induced in thearmature. The reversed armature voltage will reset the timing circuit aswell as activate the inhibitor circuit which shorts the timing circuitcapacitor. As a result, the coils will deenergize, opening the driveresistor contacts and eliminating the short across the drive resistors.The low reverse motor voltage, which is acting as a decelerator for theexisting forward motion of the armature, will permit the gradualdeceleration of the forward motion of the armature without high torque.Once forward armature motion has slowed to a near-zero speed, theinhibiting circuit will deactivate, permitting the timing circuit tooperate. At the end of the time delay, the timing circuit will begin toconduct, energizing the drive resistor coils, which will again close thedrive resistor contacts, shorting the drive resistors, and permittingmaximum reverse voltage across the motor.

A purpose of this invention is to control deceleration of a motorarmature when reversing the voltage applied across the motor. Anotherpurpose of this invention is to avoid dangerous decelerating torque bylimiting the amount of reverse voltage applied across the motor duringplugging. Yet another purpose of this invention is to delay theapplication of maximum reverse voltage across the motor until thearmature has begun to move in the desired direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and its numerous objectsand advantages will become apparent to those skilled in the art byreference to the following detailed description and the accompanyingdrawings.

FIG. 1 is a schematic diagram of the electrical circuit of a motorutilizing the control apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, an electrical circuit which embodies the inventionis shown. The armature 10 and field coil 11 represent parts of aconventional truck motor connected across the positive and negativesides 13 and 14 of poWer supply 12. Drive resistors 15 and 16 areconnected in series between field coil 11 and armature 10. The opposedends of field coil 11 are adapted to be connected to the drive resistors15 and 16 and to the positive side 13 through either the normally opencontact 32A and the normally closed contact 34B or through the normallyopen contact 34A and the normally closed contact 32B.

The arrangement is such that the pairs of contacts 32A-34B and 34A-32Bcan establish opposed directional circuits. Closing of the contact 32Aconditions the motor to drive in a forward direction, and opening of thenormally open contact 32A and the closing of the normally open contact34A reverses the voltage on the field coil 11 whereby the motor drivesthe truck in reversed direction. Contacts 32A and 32B are controlled byenergizing coil 32 and contacts 34A and 34B are controlled by energizingcoil 34. When coil 32 is energized, normally open contact 32A will closeand normally closed contact 32B will open. When coil 34 is energized,normally open contact 34A will close and normally closed contact 34Bwill open. The deenergizing of coils 32 and 34 will open each coil'snormally-open contact (32A,34A) and close its normally-closed contact(32B,34B) respectively. The energization of coils 32 and 34 arecontrolled by switch SW-F and switch SW-R respectively.

Drive resistors 15 and 16, which are, as mentioned earlier, connected inseries between field coil 11 and armature 10, control the motor speed.Control of the total resistance between the field coil 11 and thearmature 10 will control the motor speed. In such a manner, varying theresistance between the field and armature will permit the motor speed tobe varied also.

To accomplish this goal, each drive resistor 15 and 16 has a normallyopen contact 36A and 38A. respectively, connected in parallel acrosseach respective drive resistor. When contact 36A is closed, driveresistor 15 is short-circuited and the total resistance between fieldcoil 11 and armature 10 drops and motor speed will increase to a secondspeed. When contact 38A is closed, drive resistor 16 is shorted,minimizing resistance between field coil 11 and armature 10 and motorspeed will increase to a third speed.

For closing contacts 36A and 36B, coils 36 and 38 are utilized. Whencoils 36 and 38 are energized, contacts 36A and 38A close and when coils36 and 38 are deenergized, contacts 36A and 38A will reopen. Theenergizing of coils 36 and 38 are controlled by switches SW-1 and SW-2and are further controlled by the operation of timing circuit 17.Switches SW-1 and SW-2 may be connected for sequential operation withswitches SW-F and SW-R as diagrammatically indicated at 41. Timingcircuit 17 is a conventional uninjunction timing circuit such as isdisplayed in General Electric's SCR Manual, 5th Edition, page 218.

The operation of the motor in switching from its lowest speed to higherspeeds occurs as follows: During operation in the lowest forward speed,switch SW-F is closed, coil 32 is energized, contacts 32A and 34B areclosed, and field coil 11 drives the motor in a forward direction. Bothdrive resistors 15 and 16 are in the circuit, and armature 10 operatesat its lowest speed. When a higher speed is desired, switch SW-1 isclosed, activating timing circuit 17. At the end of the time delay,typically 0.3 seconds, coil 36 is energized, closing contact 36A which,in turn, shorts out drive resistor 15, decreasing the total driveresistance and increasing the motor speed to a higher level. A timedelay is interposed between the closing of switch SW-1 and theenergizing of coil 36 by timing circuit 17 to prevent a too rapidacceleration from rest to the higher forward speeds. By requiring a timedelay between speeds F and 1, it is ensured that the motor will beoperating at speed F before acceleration to speed 1 is attempted. Whenacceleration to the highest speed 2 is desired, switch SW-2 is closed.As the timing circuit 17 is still conducting, coil 38 energizes withoutdelay, closing contact 38A which minimizes the drive resistance andpermits the motor to accelerate to maximum speed.

Plugging occurs when switching from a forward speed to a reverse speedis desired. By opening switch SW-F and closing switch SW-R, contact 32Aand contact 34B will open and the contact 34A and contact 32B willclose, reversing the polarity of field coil 11 and the direction of themotor. However, the armature 10 will continue to rotate in the forwarddirection due to momentum and plugging will commence. Switches SW-F andSW-R may be interconnected for mutually exclusive operation asdiagrammatically indicated at 40.

To reduce the amount of torque imparted due to the plugging effect, thisinvention contemplates the temporary reduction of the reverse motorvoltage to reduce the severity of plugging. By reversing the polarity ofthe field coil 11 while the motor is operating in a forward direction, aback EMF of reverse voltage polarity is induced across the armature 10.When a positive voltage is applied at junction 10b, timing circuit 17 isreset and the inhibitor circuit 18 is activated. The inhibitor circuit18 is connected across the armature 10 and is comprised of anoptocoupler 19 connected in series with a diode 20 and a resistor 21.The cathode of the diode 20 is connected at the positive armatureterminal 10a. The optocoupler 19 is comprised of a light-emitting diode22 and a transistor 23. The transistor 23 is connected across the timingcircuit capacitor 24 with the transistor collector connected at thepositive capacitor terminal.

During forward acceleration of the armature 10 terminal 10a will bepositive with respect to terminal 10b, the inhibitor circuit 18 will benonconducting due to the presence of diode 20 and the timing circuit 17will operate as described above. However, when field coil 11 polarity isreversed, a positive voltage is applied at terminal 10b and detected bythe attached inhibitor circuit 18. In response to this positive voltage,light-emitting diode 22 begins to conduct, turning on transistor 23which shorts out timing capacitor 24 and inhibits the reset timingcircuit 17 from operating. The timing circuit 17 will remainnon-conducting while transistor 23 is powered, open-circuiting coil 36and coil 38. Coil 36 and coil 38 will then deenergize, opening contacts36A and 36B, and removing the short across drive resistors 15 and 16.Drive resistors 15 and 16, in turn, reduce the reverse motor voltageand, in such manner, reduce the reverse torque imparted to the armature10 to permit smooth deceleration of the truck. As decelerationcontinues, the induced voltage across the armature decreases.

When the truck has reached near-zero speed, the voltage across thearmature is insufficient to allow light-emitting diode 22 to powertransistor 23. The short across timing circuit capacitor 24 is removedand the inhibiting of timing circuit 17 ends. At the end of thedesignated time delay, timing circuit 17 will begin to conduct. Coils 36and 38 will energize, closing contacts 36A and 38A, shorting driveresistors 15 and 16 and permitting maximum reverse voltage to be appliedto the armature 10. At the time at which maximum reverse torque isapplied, the truck will have begun to accelerate in a reverse directionand the change will be much less abrupt. The near-zero speed at whichinhibitor circuit 18 deactivates may be adjusted by varying the value ofresistor 21.

Thus, there has been described and illustrated here apparatus forinhibiting high torque during the plugging of a series-resistorcontrolled motor. However, those skilled in the art will recognize thatmany modifications and variations besides those specifically mentionedmay be made without departing substantially from the scope and spirit ofthe present invention. Accordingly, it should be clearly understood thatthe form of the invention described herein is exemplary only and is notintended as a limitation on the scope of the present invention.

What is claimed is:
 1. Apparatus for inhibiting high torque whileapplying reverse voltage to an oppositely directed operating motorincluding a field coil and an armature, a power circuit, and means forconnecting said motor in said power circuit for driving in either ofopposite directions, comprising:at least one drive resistor coupled inseries with said field coil, a shorting contact connected in parallelwith each said at least one drive resistor; an energizing relay coil foreach said shorting contact; means for energizing said relay coil; meansfor deenergizing said relay coil in response to a change in polarity ofsaid field coil; and means for reenergizing said relay coil in responseto a low voltage across said armature.
 2. Apparatus of claim 1 whereinsaid at least one drive resistor is connected between said field coiland said armature.
 3. Apparatus of claim 1 wherein said means fordeenergizing said relay coil in response to a change in polarity of saidfield coil further comprises a capacitor, means for shorting saidcapacitor in response to the change in polarity of said field coil, andmeans for open circuiting said relay coil.
 4. Apparatus of claim 3wherein said means for shorting said capacitor in response to a changein polarity of said field coil comprises an opto-coupler, saidopto-coupler including a light-emitting diode and a transistor, saidtransistor being connected across said capacitor, said light-emittingdiode having a polarity that renders it activated by a reverse voltageacross said armature to power said transistor for shorting saidcapacitor.
 5. Apparatus of claim 1 wherein said means for reenergizingsaid relay coil in response to said low voltage across said armaturefurther comprises a timing circuit for delaying reenergization of saidrelay coil for a specified time period after said low voltage acrosssaid armature has been detected.
 6. Apparatus of claim 1 furthercomprising a timing circuit for delaying energization of at least one ofsaid at least one relay coil.
 7. Apparatus of claim 1 wherein said meansfor reenergizing said relay coil in response to said low voltage acrossthe armature includes a resistor to control the magnitude of thereenergizing voltage.
 8. Apparatus for inhibiting high torque whileapplying reverse voltage to an oppositely directed operating motorincluding a field coil and an armature, a power circuit, and means forconnecting said motor in said power circuit for driving in either ofopposite directions, comprising:at least one drive resistor coupled inseries with said motor; a shorting contact connected in parallel witheach said at least one drive resistor; an energizing relay coil for eachsaid shorting contact; means for energizing said relay coil; means fordeenergizing said relay coil in response to a change in polarity of saidfield coil; and means for reenergizing said relay coil in response to alow voltage across said armature; wherein said means for deenergizingsaid relay coil in response to a change in polarity of said field coilfurther comprises a capacitor, means for shorting said capacitor inresponse to a change in polarity of said field coil, and means for opencircuiting said relay coil.
 9. Apparatus of claim 8 wherein said meansfor shorting said capacitor in response to a change in polarity of saidfield coil comprises an opto-coupler, said opto-coupler including alight-emitting diode and a transistor, said transistor being connectedacross said capacitor, said light-emitting diode having a polarity thatrenders it activated by a reverse voltage across said armature to powersaid transistor for shorting said capacitor.
 10. Apparatus forinhibiting high torque while applying reverse voltage to an oppositelydirected operating motor including a field coil and armature, a powercircuit, and means for connecting said motor in said power circuit fordriving in either of opposite directions, comprising:at least one driveresistor coupled in series with said motor; a shorting contact connectedin parallel with each said at least one drive resistor; an energizingrelay coil for each said shorting contact; means for energizing saidrelay coil; means for deenergizing said relay coil in response to achange in polarity of said field coil; means for reenergizing said relaycoil in response to a low voltage across said armature; wherein saidmeans for reenergizing said relay coil in response to said low voltageacross said armature comprises a timing circuit for delayingreenergization of said relay coil for a specified time period after saidlow voltage across said armature has been detected.